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Port 391 (synotics-relay)

IANA assigns port 391 to synotics-relay: SynOptics SNMP Relay Port.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
synotics-relay TCP SynOptics SNMP Relay Port - -
synotics-relay UDP SynOptics SNMP Relay Port - -

Check what is listening on port 391

Linux: is anything listening on TCP 391?
sudo ss -tlnp 'sport = :391'
Linux: UDP 391
sudo ss -ulnp 'sport = :391'
Linux / macOS: which process owns the port
sudo lsof -nP -i :391
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':391 '
Windows (Command Prompt)
netstat -ano | findstr :391
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 391 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 391
From Windows
Test-NetConnection HOST -Port 391
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 391

Block port 391 in the firewall

ufw
sudo ufw deny 391/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 391 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 391 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 391' -Direction Inbound -Protocol TCP -LocalPort 391 -Action Block
ufw
sudo ufw deny 391/udp
iptables
sudo iptables -A INPUT -p udp --dport 391 -j DROP
nftables
sudo nft add rule inet filter input udp dport 391 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 391' -Direction Inbound -Protocol UDP -LocalPort 391 -Action Block

Related ports

389LDAP390uis392synotics-broker393meta5

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.